1. Field of the Invention
The present invention relates to electrical circuit elements; in particular, the present invention relates to circuit elements for rectifying and limiting current from an alternating current (AC) source.
2. Discussion of Background
Except for devices that are solely battery operated, most electronic devices receive power using alternating current (AC). FIG. 1A illustrates a graph of voltage versus time of an alternating current, meaning that the current reverses the direction of flow. So-called household current varies between approximately xc2x160 volts and does so at approximately 60 times per second. Unfortunately, since an AC signal will damage many electronic devices, this signal must be rectified to direct current (DC). Direct current does not change direction, although it may change amplitude as seen in FIG. 1B.
Diodes, solid state circuit elements that conduct current in forward bias but not in reverse bias, commonly are used in circuits to rectify an alternating current (AC) signal. For half-wave rectification, as illustrated in FIG. 1B, a single diode can exclude negative voltages, leaving only positive voltages. For full-wave rectification, as illustrated in FIG. 1C, two or four diodes are used.
Diodes have current limit ratings based on the maximum amount of current that can be handled. If the current limit is exceeded, the diode will no longer function as a rectifier, but as a straight wire conductor. During this xe2x80x9cshortedxe2x80x9d state, full wave AC will pass through the diode into the targeted product. As a result, many of the costly integrated circuits and other electronic components will be damaged or destroyed. More importantly, a xe2x80x9cshortedxe2x80x9d diode poses serious safety risks. For example, power supplies often use capacitors to filter the rectified signal to reduce any possible hum or buzzing noise. If full wave AC is allowed to pass to the filter, the capacitors may explode.
Various circuits have been devised to reduce the potential threat of a xe2x80x9cshortedxe2x80x9d diode. These circuits use other circuit elements to open the circuit if a diode is xe2x80x9cshorted.xe2x80x9d Fuses are commonly placed in circuits using diodes so that the fuse will open the circuit before the current limit on the diodes is reached. Also, xe2x80x9cshuntxe2x80x9d circuits are used which purposely blow a fuse if a diode has xe2x80x9cshorted.xe2x80x9d Although these circuits protect the device from damage in the event that a diode xe2x80x9cshorts,xe2x80x9d additional circuit elements increase overall costs. Moreover, a myriad of fuses with various current ratings must be inventoried to be prepared for differing applications. Therefore there is a need for a single circuit element that not only rectifies an AC signal, but also opens the circuit when a programmed current limit is reached.
According to its preferred embodiments and briefly described, the present invention is a solid state rectifying fuse (SSRF), or xe2x80x9cGibboneyxe2x80x9d diode, after the inventor. The SSRF rectifies an incoming AC signal and opens the circuit if a programmed maximum current level is reached. The anode or cathode depending upon the particular application is designed with a plurality of conductor strips in electrical contact with the P or N region, respectively. Conductors are composed of a material that melts open if the current limit is exceeded. Conductors are designed to have a lower total current handling capacity than that of the PN junction so that the current is reduced or terminated before the PN junction can xe2x80x9cshort.xe2x80x9d Depending on the desired maximum current of the SSRF, the current limit can be adjusted downward by burning out or severing a portion of the conductors. In this sense, it is a xe2x80x9cprogrammablexe2x80x9d diode.
A major feature of the present invention is the ability to not only rectify an incoming signal, but also to open the current path if a programmed current limit is exceeded. Unlike typical diodes that xe2x80x9cshortxe2x80x9d or function as a straight wire connection if the current limit is exceeded, the present invention incorporates a plurality of conductors into the diode that open the current path if a programmed current limit is reached. In this way, the SSRF is both a diode and a programmable fuse in one circuit element.
A major advantage of the present invention is a resulting increase in safety not only for a person handling the circuit but also to reduce the possibility of damaging circuit components. Unlike typical diodes that may pass a full wave AC signal through the circuit if the current limit is reached, the present invention will open the current path to prohibit the passage of AC to other components in the circuit. Moreover, if the programmed current limit is exceeded, persons handling the circuit will not be exposed to AC.
Another important advantage of the present invention is lower overall costs. The costs added by fuses, fuse holders and the like are not necessary with circuits using the SSRF since it also functions as a fuse. Moreover, since the SSRF may be programmed for various current limits, the number of diodes that manufacturers must inventory is significantly reduced.
Another important feature of the present invention is the ability, to program for a specific current handling limits. A SSRF may be manufactured to open the current path when a particular current level is reached. This current limit may be adjusted downward to a desired current limit by burning out or severing a portion of the conductors.